1. Field of the Invention
The present invention relates to electric heating sheets and, in particular, to flexible electric heating sheets of the type used to heat articles such as seats in automobiles, trucks, other vehicles, aircraft and the like.
2. Related Art
Planar heating elements have been used for such applications as automobile seat heating, ceiling heating, underfloor heating, wood/panel heating, motor vehicle mirror and personal cushion heating. Automobile seats have been heated, for example, by flexible elements which are fabricated by bonding an insulated resistor wire to the internal surface of a fabric or leather covering of the seat. The wire is typically made of nickel/chromium or another suitable high-resistance metal. Heat is generated by power dissipated during the passing of current through the wire. The power dissipated is given by the expression: P=I.sup.2 R=VI where P is the power dissipated (watts), R is the resistance of the heating element (ohms), I is the current running through the element (amperes) and V is a voltage drop in the heating element (volts). A problem arises in that the heating surface of the wire occupies generally only one to five percent of the surface area required to be heated. Because of the high power required to heat the entire surface from the narrow wire, hot spots are created on the surface which may cause premature deterioration of the fabric or leather covering.
Attempts have been made to ameliorate the aforementioned problem. For example, U.S. Pat. No. 5,229,582 to Graham, issued Jul. 20, 1993, discloses a flexible heating element constructed by securing a flexible layer of conductive material constituting an electric heater to a flexible sheet. The layer of conductive material is connected to an electrical supply means by at least one copper strip (column 2, line 22) electrode that has an embossed surface with protuberances which are said to enhance direct contact of the electrode with the layer of conductive material. A thermoplastic polymer-based adhesive at the interface between the embossed surface and the conductive layer secures the latter to the electrode. The protuberances are provided to penetrate the adhesive layer to provide an improved electrical contact between the electrode and the conductive material.
U.S. Pat. No. 5,629,073 to Lovell, issued May 13, 1997, which discloses a medium-temperature conductive resistive article made from a composition including graphite suspended in a high-temperature polymer base activator with water. The conductive resistive composition can be applied as a layer to a fabric-like substrate in order to provide a resistive temperature adjustable heating element for the fabric. Conductive strips of copper foil "as well as many other types of electrodes" (column 3, line 15) are connected to the heating element as electrodes.